This invention relates to ophthalmic uses of lasers, in particular the methods for analysis and therapy of the eye utilising laser induced ultrasound.
There is considerable interest in medical applications of ultrasound and photo acoustic waves, sometimes known as laser-generated ultrasonic waves. This is because, in contrast to interrogating techniques such as X-ray imaging and scintigraphy, ultrasound offers the possibility of interrogating a subject with no harmful side effects. Ultrasound imaging of cardiac structures, the vascular systems, the foetus and uterus, abdominal organs such as the liver, kidneys and gall bladder is known. Conventional ultrasound measurements of the eye, using piezoelectric devices to generate ultrasound, are known, and are employed to conduct dimensional measurements of the position of various interfaces within the eye. High resolution measurements are needed for measurements of the front portion of the eye. However, the high frequency ultrasonic signal employed for these measurements do not penetrate the retina and beyond, and hence a lower frequency piezo electric transducer is employed in order to measure these larger dimensions. This has the disadvantage of giving rise to a loss of relative distance information. Additional disadvantages are that two piezoelectric transducers are required, and that separate measurements are necessitated.
The present invention provides an improved method for analysing an eye using laser radiation to generate ultrasound at desired locations in the eye.
According to the invention there is provided a method for analysis and/or therapy of an eye comprising the step of directing laser radiation into the eye, the characteristics of the laser radiation being suitable to generate ultrasound at desired locations within the eye.
It is possible to generate ultrasound in this way at numerous locations within the eye using a single source. In particular, ultrasound may be generated at the back of the eye, at the sclera, choroid, retina or vitreous humour, or at the front of the eye, i.e. the cornea, lens, iris and ciliary body and the aqueous humour, by a single laser pulse. To reflect ultrasound from the retina requires the use of much lower frequency ultrasound signals than needed to make high resolution dimensional measurements of the front portion of the eye. Conventionally, this necessitates the use of two piezoelectric transducers.
The generated ultrasound may be detected by ultrasonic transducer means connected to the eye by an acoustic coupling medium. The method may comprise an analysis of the eye in which the time taken for the generated ultrasound to propagate from the location at which it is generated to the ultrasonic transducer means, or a time value closely related thereto, is measured, and information concerning the eye is derived therefrom. The information concerning the eye may comprise the spatial dimensions of the eye or of one or more components of the eye. The information concerning the eye may also comprise opacity measurements within the eye.
The laser radiation may be directed to the eye substantially along an axis which is substantially parallel to the optical axis of the lens. The term xe2x80x9csubstantially along an axisxe2x80x9d is intended to encompass instances where a laser beam does not propagate along a single axis due to divergence of the beam or focussing of the beam, but, nonetheless, an xe2x80x9caveragexe2x80x9d beam path is described by a single axis.
Alternatively, the laser radiation may be directed into the eye substantially along an axis which is substantially non-parallel to the optical axis of the lens. This enables inter alia xe2x80x9coff axisxe2x80x9d White-White measurements to determine the dimensions of the ciliary sulcus.
The point of entry of the laser radiation into the eye may be varied. In this way, an image of the eye, or one or more components of the eye, may be constructed from measurements made at a plurality of points of entry of the laser radiation into the eye. In this way, a B-scan may be performed. The variation of point of entry of the laser radiationxe2x80x94which is preferably accomplished by traversing a laser beam vertically or horizontally across the eyexe2x80x94may be combined with xe2x80x9coff-axisxe2x80x9d measurements of the type described above.
A plurality of ultrasonic transducers may be employed to provide a plurality of response waveforms, and the waveforms may be processed by adding appropriate time delays thereto so as to provide position sensitive measurements of the generated ultrasound. The laser radiation may emanate from a source, and the plurality of ultrasonic transducers may comprise a plurality of substantially annular transducers arranged substantially concentrically with respect to the source. Alternatively, the plurality of ultrasonic transducers may comprise an array of discrete ultrasonic transducers, and the responsive waveforms provided thereby may be processed to provide a three dimensional image of the eye, or of one or more components of the eye.
Information concerning the lens may be derived from the generated ultrasound, such as the size and position of the lens.
Information concerning light scattering sources present in the eye may be derived from the generated ultrasound.
The opacity of the lens may be measured.
The presence of flare and cells and like scattering sources in the vitreous humour and/or aqueous humour may be detected.
Preferably, a device having ultrasonic transducer means and a source for directing the laser radiation to the eye combined in a single housing is used. Such devices are known from, or may be adapted from devices known from, British Patent Application GB 2 212 920 A and International Publication WO 94/28804, the contents of which are hereby incorporated by reference.
The acoustic coupling medium may comprise a saline solution or a gel.
The invention provides a method for treating pathological lesions of the layers of the wall of the eye in which tissue destruction is effected by the laser radiation. In this way, glaucoma may be treated by effecting destruction of the ciliary body.